Vacuum casting



April 7, 1959 c. w. HANKS ETAL VACUUM CASTING 2 Sheets-Sheet l MFLT5706,@

Filed June 1l, 1957 7 5N K Mw H. 4 w d 55 E.: Ll r. C

VACUUM CASTING Filed June 11, 1957 2 Sheets-Sheet 2 E www PUMP l(ff/,m55 MHA/W5 7' A mp 6457' waar wif (uw 15 da?, /w//vr uw wa# R, 5Mr/f, u?.

INVENTOM United States Patent() VACUUM CASTING Charles W. Hanks andCharles dA. Hunt, Orinda, and

Hugh R. Smith, Jr., Berkeley, Calif., assignors to Stauffer ChemicalCompany, New York, N .Y., a corporation of Delaware Application June 11,1957, Serial No. 664,982 7 Claims. (Cl. 22-57.2)

This invention relates to the art of casting ingots, rods and the likeby drip-melting compacted or solid melt stock into an annular, cooledmold from the bottom of which resolidied material is withdrawncontinuously or semi-continuously to maintain a relatively constantlevel of molten material within the mold.

A crucial need has arisen for improved cold-mold casting means suitablefor the large-scale commercial processing of metals, such as titaniumand tantalum, that have intense chemical activities when in the moltenstate. In the production of such metals, the initial ingots may containimpurities, voids, porosities and surface irregularities. They may beporous compacted masses. Melting and casting is required for variouspurposes, including purication and the production of sound ingots, rodsand the like, Ifree of voids, porosities, and surface irregularities,that are suitable for subsequent metal-Working and fabricationprocesses. Conventional melting and casting techniques used with lessactive metals are not feasible with the intensely active metals, whichin the molten state attack ordinary crucibles and molds.

Heretofore, the most common technique for large-scale casting of theintensely active metals utilizes a consumable electrode and awater-cooled annular copper mold. An electric arc is established betweena lower end of the consumable electrode and material within the mold.The arc heats and gradually melts the consumable electrode, which dripsinto a molten pool of metal within an upper portion of the annular mold.The metal held within the mold solidifies from the circumference inward,forming a bowl or skull of solidified metal that supports the pool ofmolten metal. Thus, contact between the intensely active molten metaland the copper mold is kept to a minimum. As the metal solidifies, it iswithdrawn continuously or semi-continuously from the bottom of theannular mold to maintain a substantially constant level of molten metalwithin the mold.

The consumable-electrode arc-melting technique has several signiticantadvantages. The rate of heat supply to the consumable electrode shouldbe just suiiicient to maintain the desired rate of drip melting. Therate of heat supply to the pool of molten metal within the mold shouldbe just suflicient to maintain a molten pool of adequate size for theformation of sound castings. An incorrect division of heating energybetween the two electrodes, or an incorrect ratio between the heatingrates and the rate of cooling within the mold, may result in either adeficiency or an excess in the pool of molten metal. A deficiency causesunsound castings, containing voids, porosities and surfaceirregularities. An excess causes accelerated chemical activity betweenthe molten pool and the mold, rapidly destroys the mold, andcontaminates the cast metal.

Where arc-melting is employed, the melting operation and the castingoperation are interdependent, since the same electric arc supplies heatto both electrodes in a ratio that is not controllable. Hence,separation of the Significant variables affecting the separateoperations is 2,880,483 4Patented Apr. 7, 1959 impossible, bothexperimentally and operationally. Further, the operation must beperformed batch-wise because of the relatively large amount ofspattering of metal which occurs as the consumable electrode melts underthe influence of enormous localized current concentrations in the arcbetween the consumable electrode and the molten metal surface atop thecast ingot. Also, the consumable electrode must extend into the moldwell past guiding supports of the electrode. The danger of shortcircuitsto the copper walls of the mold is considerable, and explosions occurfrequently in large-scale operations because of electric currentconcentrations at the side walls of the mold that melt the mold in spotsand allow cooling liuid to contact molten metal.

Briefly stated, in accordance with certain aspects of this invention, animproved casting process is provided for drip-melting melt sto-ck into amolten pool maintained on the top of a resolidifying ingot or rod withina cooled mold. This process is carried out in a high vacuum, which forpurposes of this patent application is defined as an absolute pressurenot exceeding approximately one micron of mercury. Both the consumablemelt stock and the molten pool are anodes, and are heated by highvoltageelectron bombardment from one or more cathodes. Either anode (or bothanodes) can be bombarded and heated at will, and at regulated rates, byregulating the electron ilow from the cathode or cathodes to the anodes.The cathodes can be made of metals or materials different than theconsumable melt stock and the cast ingot Without danger of contaminationof the cast ingot due to this dissimilarity. Unlike arc meltingprocesses, the cathodes and cathode structures do not melt, nor do theytouch the anodes at any time in the melting process. Their sole purposeis to supply electrons for bombarding the anodes. v

Because of the high vacuum that is maintained, the current between theelectrodes is an essentially electronic current, which heats the anodesby conversion of the kinetic energy of the electrons into thermal energyas the electrons strike the anodes. Thus, the nature of the electricaldischarge is very different from that of low-voltage discharges, such aselectric arcs and the glow discharges that prevail at higher pressures.Metal vapors and other gaseous matter emanating from the anodes maypartially or Wholly neutralize the electronic space charge.

Preferably, the consumable electrode of melt stock is a vertical rodpositioned above and vertically almed with an annular mold containingthe cast ingot with a molten pool of metal at its top end. In oneembodiment of the invention a single annular cathode is employed forbombarding both the melt stock and the molten pool. In anotherembodiment, two annular cathodes are employed, one above the other, toprovide better control of the heating ratio for producing ingots androds of a higher quality and with better surfaces. In both casesfocusing electrodes hereinafter described are employed for confining theelectron beams to desired paths.

In the improved process, the heating rates can be controlled accuratelyby regulating the electron current and, particularly with the twocathode apparatus, the rate of heat supply to each anode can beindependently regulated to maintain any desired heating ratio. Theprocess can be operated continuously for long periods of time, withoutexcessive splattering of the molten metal. The danger of short-circuitsand explosions is substantially eliminated and the process operates witha high degree of safety.

According to another of its aspects, this invention provides an improvedzone-refining process that is particularly useful for the purificationof the intensely-active metals. As practiced heretofore, zone refiningof mal terials that attack hot crucibles has been limited to thelrefining of rods having suciently small diameters that Asurface tensioncan keep a floating zone of molten material attached to the rod. In thepresent invention, zone refining is accomplished by drip-melting themelt stock into a molten ypool of'jthe material being refined. Sincethere Vis a break or space between the melt stock and the molten pool,the present invention provides an improved interrupted zone refiningprocess.

As material resolidiiies at the bottom and sides of the molten pool,impurities tend to remain in the molten pool and the resolidiiedmaterial is purer than the melt stock. The process can be repeated asmany times as may be desired to effect successive purilications of a rodor ingot. Because of the relatively slow rates of drip melting andresolidilication used in the zone refining process, separate 'heating ofvthe molten pool to maintain an adequate amount of material in themolten state is essential. Furthermore, the drip melting rate and therate of heat supply to the molten pool are individually capable ofprecise regulation. The present invention makes this possible and alsoprovides additional purilication through the evolution of volatileimpurities into the high vacuum employed. With this invention, ingotsand rods of large diameter can be handled, and large-scale commercialzone-refining is made feasible, even for materials having intensechemical activity when in the molten state.

vThe foregoing and other aspects of this invention may be betterunderstood from the following description of illustrative examples takenin connection with the accompanying drawings. The Vscope of theinvention is pointed out in the appended claims.

In the drawings:

Fig. 1 is a somewhat schematic vertical section of an improved castingapparatus;

Fig. 2 is a horizontal section taken along the line 2-2 of Fig. 1; and

Fig. 3 is a somewhat schematic vertical section of another improvedcasting apparatus.

Referring now to Fig. l of the drawings, the casting process is carriedout within a closed chamber ll that is evacuated to a high vacuumthrough a passage 2 connected to a conventional vacuum pump (not shown).Melt stock in the form of a rod or ingot 3 is fed into the vacuumchamber through a conventional vacuum seal d. A suitable electricalconnection 5 is provided for maintaining rod 3 at a reference electricpotential herein called circuit ground. Circuit ground usually isestablished by a low-resistance electrical connection to the earth, andfurnace. or building framework for safety reasons.

Rod 3 of the melt stock forms a consumable electrode disposed verticallywith its lower end above and in vertical alinement with the center of anannular copper mold 6. Mold 6 is surrounded by a water jacket 7 that isconnected to an inlet pipe 8 and an outlet pipe 9 arranged for thecirculation of water or other liquid through the water jacket forcooling mold 6. Both the top and the bottom of annular mold 6 are open.Material melted from rod 3, as hereinafter explained, drips into the topof mold 6 and forms a pool of molten material 10. As heat flows from themolten material yto water-cooled mold 6, the molten material solidiiiesinward from the circumference and upward from the bottom of the moltenpool, and thus forms an ingot of resolidiied material 11 having a bowlor skull at its top which contains the pool of molten material. andminimizes Contact between the molten material and mold 6.

As melting and resolidiication proceed, rod 3 of melt stock iscontinuously or semi-continuously moved downward Vto keep the lower endofV rod 3 Vat a substantially constant level within the vacuum chamber,and the ingot 11 of resolidiiied material is continuously orsemi-continuously moved downward to maintain a substantially constantlevel of molten material within the upper portion of moldv 6. The ingot11 of resolidiiied material is withdrawn through the open bottom of mold6, and may be withdrawn from the vacuum chamber through aconventional'vacuum seal 12.

Thus the melt stock is continuously melted and cast into a new ingot ofresolidiiied material. The melt stock may contain impurities, voids,porosities, and surface irregularities. It may be a porous compactedmass of material formed, for example, by pressing together powdered orpelletized material. If the process is properly regulated, the ingot il.of resolidilied material will be a sound casting, relatively free ofvoids, fissures, porosities and surface irregularities, Rod 3 maycomprise a mixture of powders or pellets of different materials andingot 11 an alloy of these materials. The process may also be used forpurification or refining, to remove volatile impurities that are`evolved from the molten material into the high-vacuum, and to removeimpurities that tend to remain in the molten pool as the cast materialresolidifies.

For accomplishing the purposes of this invention., heat must be suppliedto the consumable electrode 3 andthe pool of molten material lil. Heatis supplied to the ylower end of electrode 3 for the purpose of meltingit. The rate of heat supply to the consumable electrode controls therate of melting. Heat must be supplied to lthe molten material it) tomaintain a molten pool of adequate size for the production of soundcastings. The rate of heat supply to pool l0, in relation to otherfactors including the rate of heat transfer to mold 6, controls thedepth yof the molten pool.

if the rate at which heat is supplied to pool 1d is too low, a moltenpool of adequate size will not be maintained at the top of ingot 1l andeach new drop of molten material that drips from electrode 3 will splashonto the top of ingot lll and there solidify quickly and irregularly.The end result will be an unsound casting, which may contain voids,fissures, surface irregularities and other undesirable characteristics.

ln accordance with the present invention, the consumable electrode 3 andthe pool of molten metal lll are both heated by high-voltage electronbombardment. Electrode 3 is maintained at circuit ground throughelectrical connection 5, and pool lil is maintained at circuit groundthrough the grounded copper mold. rl`he lower end of rod 3 and the uppersurface of pool 10 constitute anodes for the electron bombardmentsystem.

An annular cathode is formed from a horizontal circular loop of tungstenwire 13. The cathode is somewhat larger in diameter than either of rod 3and ingot l1, as is shown in the drawings. The cathode is in verticalaxial alinement with rod 3 and ingot 11 and is disposed below the bottomof rod 3 and above the top of mold 6, so that electrons emitted by thecathode can bombard both of the two anodes. The two ends 13 and 13" oftungsten wire 13 pass through insulators 14 and 15 through a side wallof vacuum enclosure 1. The insulators are protected from thecondensation of metallic vapors by suitable means such as shields 16 and17.

A. transformer 13, having its primary connected to any suitable A.C.supply and having its secondary connected across the two ends 13 and 13of wire 13, supplies to the cathode sutcient current to heat the sameand pro'- duce thermionic emission of electrons from the cathode. A DC.supply 19 connected between circuit ground and the secondary oftransformer 18 maintains the cathode at a negative potential relative tothe two anodes, so that electrons emitted from the cathode bombard thelower end of rod 3 and the upper surface of molten pool 1l). Theelectrons that bombard the lower end of rod 3 gradually melt theconsumable electrode, and melted metal drips into pool ll as indicatedby drop 20. The electrons that bombard the upper surface of pool 10 keepan adequate amount of the material in a molten state to assure theproduction of sound ingots.

An important function is performed by the annular focusing electrode 2.1which substantially surrounds cathode 13, as shown.v A metal bracket 22helps to support cathode 13 and also provides an electrical connectionbetween cathode 13 and focusing electrode 21, in consequence of whichthe focusing1 electrode is maintained at cathode potential. The annularfocusing electrode 21 has a channel-shaped cross-section, as shown,which opens inwardly and shields the top, outer circumference and bottomof the annular cathode. Electrode 21 is supported by any suitable means,such as straps 23 and 24 suspended from the top of vacuum enclosure 1and insulated therefrom by insulators 25 and 26. The insulators areprotected from the condensation of metallic vapor by suitable means suchas shields 27 and 28.

The focusing electrode 21 is maintained at cathode potential and becauseof the strong electric field existing between cathode and anode there islittle tendency for the electrons emitted by the cathode to travel tothe focusing electrode. Instead, most of the electrons emitted by thecathode 13 are directed inwardly from the cathode, and then upwardlytoward the bottom of consumable electrode 3 and downwardly toward thetop surface of molten pool 10. By this means the electrons areconcentrated where they are needed and there is no substantialbombardment of mold 6, the sides of rod 3, and other parts that shouldnot be bombarded.

Because of the high vacuum maintained within vacuum chamber 1, thecurrent between cathode 13 and the two anodes is essentially electronicin character. The impedance of the space-discharge path is high and arelatively high-voltage, low-current discharge is maintained. This typeof electron discharge permits regulation and control of the thermalpower developed to a degree of precision that would be utterlyimpossible with an electric arc or similar low-impedance discharge. Inthe immediate vicinity of the two anodes the melting and the moltenmetal has sufficient vapor pressure to provide an ionizable vapor, whichto some extent neutralizes the electronic space charge and provides alow-resistance discharge region immediately adjacent to each anode. Thishas the beneficial effect of spreading the discharge rather uniformlyover both of the two anodes, and substantially eliminates localizedcurrent concentrations or hot spots that are a cause of unequal heating,metal spatterng, and other diiiiculties in the prior-art dripmeltingtechniques utilizing electric arcs.

The total power input to the heating system can be controlled byregulating the current supplied by D C. supply 19, and thus regulatingthe total electron bombardment current. The thermal power developed isequal to the square of the electron current times the resistance of thespace-discharge path. The high vacuum discharge path has a highresistance. The total electron current is equal to the current suppliedby supply 19. The ratio between the rate of heat supply to electrode 3and the rate of heat supply to molten pool can be regulated by adjustingthe relative distances of the two anodes from cathode 13. If the lowerend of rod 3 is raised slightly from the position shown in Fig. l, asmaller percentage of the electrons emitted by cathode 13 will bombardthe lower end of rod 3, and a larger percentage of the electrons willbombard pool 10. Consequently, the rate of heat supply to electrode 3will be reduced relative to the rate of heat supply to molten pool 10.Conversely, if the lower end of rod 3 is lowered slightly from theposition shown in Fig. l, a larger proportion of the electrons willbombard the lower end of rod 3, and the rate of heat supply to rod 3will be increased relative to the rate of heat supply to molten pool 10.By controlling the total heating energy supplied to both anodes, throughthe adjustment of D.C. supply 19, and controlling the ratio of the heatenergies supplied to the two anodes by adjusting the position of rod 3,it is evident that the heat energy supplied to each anode can beindividually adjusted to approximately any desired value.

The single-cathode arrangement shown in Fig. l works quitesatisfactorily if a moderate amount of variation as an irregularfunction of time can be tolerated in the rela'- tive distribution ofheating energy between the two anodes. If a more precise regulation ofthe energy distribution is required (for the casting of ingots havingfewer surface irregularities, or for zone refining, for eX- ample), thedouble-cathode configuration shown in Fig. 3 should be employed.

Referring to Fig. 3, the apparatus illustrated is essentially similar tothat shown in Fig. 1 except for the greater separation between theconsumable electrode 3 and molten pool 10, and the provision of twoseparate cathodes for bombarding respective Ones of the two anodes. Tosimplify and to clarify the description, parts of the Fig. 3 apparatusthat are substantially the same as corresponding parts of the Fig. lapparatus are identified by the same reference numbers.

In the Fig. 3 apparatus the upper cathode 13 and fo cusing electrode 21are substantially identical to the cathode and focusing electrode of theFig. 1 apparatus, except that in the Fig. 3 apparatus the upper cathodeis at a relatively great distance from the molten pool 10, inconsequence of which most of the electrons emitted by cathode 13 bombardand heat the lower end of consumable electrode 3. Consequently, the rateof melting of electrode 3 is substantially a direct function of theinput power provided by D.C. supply 19 and can be precisely regulatedand controlled by regulating electrons supplied by the cathode.

A second annular cathode 29 and focusing electrode 30 are substantiallyidentical to cathode 13 and focusing electrode 21 except that the secondcathode is positioned in axial alinement with and just above molten pool10 and is spaced a relatively great distance from the lower end ofconsumable electrode 3. Consequently, most of the electrons emitted bylower cathode 29 bombard and heat the upper surface of molten pool 10.For heating cathode 29 and producing a thermionic emission of electrons,a transformer 31 has its secondary connected across the two ends 29 and29" of cathode 29 and has its primary connected to any suitable A.C.supply. A second D.C. supply 32 is connected between the Secondary oftransformer 31 and circuit ground, as shown. Supply 32 provides thelower cathode with a negative electric potential relative to the pool ofmolten metal, so that electrons emitted by the lower cathode bombard themolten pool. With this arrangement the amount of heating energy suppliedto the upper surface of molten pool 10 is a direct function of the inputpower provided by D.C. supply 32 (the square of the electron currenttimes the resistance of the space-discharge path), and consequently thesize of the molten pool can be precisely controlled and regulated byregulation of D.C. supply 32.

The two-cathode configuration shown in Fig. 3 makes it possible tocontrol with a high degree of precision both the rate of melting of rod3 and the rate of heat supplied to molten pool 10, each independently ofthe other. It is only necessary that the two cathodes 13 and 29 besufficiently separated to prevent any undesirable amount ofcross-bombardment of each anode by the more distant cathode. It has beenfound that this can be accomplished quite satisfactorily by providingbetween the two cathodes a vertical separation about equal to or greaterthan the larger of the diameters of electrode 3 and ingot 11.

With the arrangement shown in Fig. 3, cast ingots of superior qualitywith a minimum of surface irregularities can be produced easily. Thearrangement shown in Fig. 3 is also advantageous for zone refiningprocesses in which the melting rate and the size of pool 10 should bequite uniform and precisely regulated. Where the requirements are lessexacting the configuration of Fig. 1 may be employed with good results.

In a typical casting operation according to this invention, theconfiguration of Fig. 1 was operated successfully :usingB-inch diametertitanium electrodes and ingots, a f4-.inch diameter ring of 1A() inchdiameter tungsten wire as a cathode, and a D.C. supply of 7000 voltsproviding -a total input power between about and 20 kilowatts.

The configuration illustrated in Fig. 3 was successfully operated using3-inch diameter titanium electrodes and ingots, 4l-inch diameter ringsof 1/10 .inch diameter tungsten Wire as cathodes, and a D.C. supply ofabout 7000 volts for each cathode. The threshold of melting of theconsumable electrode was reached with an input power of about 5kilowatts provided by supply 19. At 7 to 8 kilowatts of input power fromsupply 19, electrode 3 melted at a rate of approximately inches perhour. Sound and smooth-surfaced cast ingots were produced under theseconditions with an input power of 12 to 15 kilowatts provided by D C.supply 32 for bombarding the moltenY pool. In addition, about l kilowattof 'alternating current power was supplied to each cathode for heatingthe cathode to produce thermionic emission.

The process for zone refining is substantially the same as the castingprocess, except that for zone refining relatively slow melting andresolidification rates are employed to increase the tendency forimpurities to remain in the molten pool as the cast materialresolidifies. In Zone relining, the melting and casting process may berepeated several times, the cast ingot forming the consumable electrodefor the following melting operation, to produce successive ingots ofincreasing purity.

The drawings are somewhat schematic, and illustrate only the essentialparts of the apparatus. In practice, heat shields are employed to reduceheat flow between the hot parts and the Vacuum chamber walls.

It should be understood that this invention in its broader aspects isnot limited to specific examples herein illustrated and described7 andthat the following claims 'are intended to cover all changes andmodifications within the true spirit and scope of the invention.

What is claimed is:

l. Casting apparatus comprising an annular mold open at its top andbottom, said mold being adapted to form a rod-like ingot of resolidiliedcast material, means for supporting a rod-like consumable electrode o-fmelt stock with a lower end above and in vertical alinement with theopen top of said mold, substantially annular cathode means disposedbetween and in vertical alinement with thek bottom of said electrode andthe top of said mold, means for enclosing and evacuating a spacecontaining the bottom of said consumable electrode, the top of said moldand said cathode means, means for heating said cathode means to producethermionic emission of electrous therefrom, means for establishing onsaid consumable electrode a positive electric potential relative to saidcathode means so that electro-ns emitted by said cathode means bombard,heat and melt the lower end of said consumable electrode, materialmelted from said consumable electrode dripping into said mold, means forestablishing on the material within said mold a positive electricpotential relative to said cathode means so that electrons emitted bysaid cathode means bombard, heat and maintain a pool of molten materialresting on top of the resolidified material within said mold, meansfoilowering said co-nsumable electrode as it melts, and means forwithdrawing solidified material from the bottom of said mold to maintaina substantially constant level of molten material within said mold.

2. Casting apparatus comprising an annular mold open at its top andbottom, said mold being adapted to form a vertical rod-like ingot ofresolidilied cast material, means for supporting a vertical rod-likeconsumable electrode of melt stock above and in vertical axial alinementwith said mold, an annular filamentary cathode of larger diameter thanthe larger in diameter of said consumable electrode and said ingot, anannular focusing electrode having an inwardly opening channel-shapedcross-section, said focusing electrode shielding the top, outercircumference and bottom of said cathode, means for keeping saidfocusing electrode substantially at cathode potential for directing theelectrons emitted by said cathode inwardly, means for enclosing andevacuating a space containing the bottom of said consumable electrode,the top of said mold, and said cathode, means for heating said cathodeto produce thermionic emission of electrons therefrom, means forestablishing on said consumable electrode a positive electric potentialrelative to said cathode so that electrons emitted by said cathodebombard, heat, and melt the lower end of said consumable electrode,material melted from said consumable electrode dripping into said mold,means for establishing on the material within said mold a positiveelectric potential relative to said cathode so that electrons emittedfrom said cathode bombard, heat and maintain a pool of molten materialresting on top of the resolidified material within said mold, means forcooling said mold so that the molten material resolidilies inwardly fromthe circumference and upwardly from the bottom of said pool, means forlowering said consumable electrode as it melts, and means forwithdrawing resolidified material from the bottom of said mold tomaintain a substantially constant level of molten material within saidmold.

3. Casting apparatus comprising an annular mold open at its top andbottom, said mold being adapted to form a rod-like ingot of resolidifiedcast material, means for supporting a rod-like consumable electrode ofmelt stock with a lower end above and in vertical alinernent with theopen top of said mold, first and second substantially annular cathodesdisposed between and in vertical alinement with the bottom of saidelectrode and the top of said mold, said first cathode being verticallyspaced above said second cathode, means for enclosing and evacuating aspace containing the lower end of said consumable electrode, the topofsaid mold, and said two cathodes, means for heating both of said twocathodes to produce thermionic emissio-n of electrons therefrom, meansfor establishing on said consumable electrode a positive electricpotential relative to said first cathode so that electrons emitted bysaid first cathode bombard, heat and melt the lower end of saidconsumable electrode, material melted from said consumable electrodedripping into said mold, means for establishing on the material withinsaid mold a positive electric potential relative to said second cathodeso that electrons emitted by said second cathode bombard, heat andmaintain a pool of molten material resting on top of the resolidifiedmaterial within said mold, means for lowering said consumable electrodeas it melts, and means for withdrawing resolidified material from thebottom of said mold to maintain a substantially constant level of moltenmaterial within said mold.

4. Casting apparatus comprising an annular mold open at its top andbottom, said mold being adapted to form a vertical rod-like ingot ofresolidilied cast material, means for supporting a vertical rod-likeconsumable electrode of melt stock above and in Vertical axial alinementwith said mold, a first annular cathode of larger diameter Ythan saidconsumable electrode, a second annular cathode of larger diameter thansaid ingot of resolidified cast material, said cathodes being disposedbetween and in vertical axial alinement with the bottom of saidconsumable electrode and the top of said mold, said second cathode beingdisposed below said first cathode and vertically separated therefrom bya distance not substantially smaller than the larger of the diameter ofsaid consumable electrode and the diameter of said ingot, means forheating both of said two cathodes to produce thermionic emission ofelectrons therefrom, first and second annular focusing electrodes eachhaving an inwardly opening channel-shaped cross-section, said rstfocusing electrode shielding the top, outer circumference and bottom ofsaid first cathode, said second focusing electrode shielding the top,outer circumference and bottom of said second cathode, means for keepingsaid rst and second focusing electrodes substantially at the samepotentials as said first and second cathodes respectively, wherebyelectrons emitted by said cathodes are directed inwardly, means forestablishing on said consumable electrode a positive electric potentialrelative to said irst cathode so that electrons emitted by said irstcathode bombard, heat and melt the lower end of said consumableelectrode, material melted from said consumable electrode dripping intosaid mold, means for establishing on the material within said mold apositive electric potential relative to said second cathode so thatelectrons emitted by said second cathode bombard, heat and maintain apool of molten material resting on top of the resolidiied materialwithin said mold, means for cooling said mold so that the moltenmaterial resolidiles inwardly from the circumference and upwardly fromthe bottom of said pool, means for lowering said consumable electrode asit melts, and means for withdrawing resolidiiied material from thebottom of said mold to maintain a substantially constant level of moltenmaterial within said mold.

5. The method of casting that comprises introducing a consumableelectrode of melt stock into a high-vacuum chamber containingthermionically electron-emissive cathode means and a mold, evacuatingsaid chamber to a high vacuum for maintaining an essentially electroniccurrent to said cathode, causing electrons emitted by said cathode meansto bombard, heat and melt said consumable electrode, causing materialmelted from said electrode to drip into said mold and there resolidfy,causing electrons emitted by said cathode means to bombard materialwithin said mold for maintaining a pool of molten material within anupper portion of said mold, regulating the electron current between saidcathode means and said consumable electrode to control the melting rate,and regulating the electron current between said cathode means and thematerial within said mold for controlling the amount of molten materialin said pool.

6. The method of casting that comprises introducing a consumableelectrode of melt stock into a high-vacuum chamber containing anelectron-emissive cathode and a maintaining a pool of molten materialwithin an upper portion of said mold, and adjusting the spacing betweensaid cathode and said consumable electrode relative to the spacingbetween said cathode and said mold for adjusting the ratio of theelectron currents lowing between said cathode and said consumableelectrode and between said cathode and the material within said mold.

7. The method of casting that comprises introducing a consumableelectrode of melt stock into a high-vacuum chamber containing lirst andsecond electron-emissive cathodes and a` mold, evacuating said chamberto a pressure not exceeding approximately 1 causing electrons emitted bysaid iirst cathode to bombard, heat and melt said consumable electrode,said consumable electrode being positioned above said mold so ReferencesCited in the file of this patent UNITED STATES PATENTS Berghaus et alDec. 3l, 1940 Herres June 2, 1953 Newcomb et al. Sept. 11, 1956 UNITEDSTATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent NoD 2,88Gylr83April 7'p 1959 Charles W Hanks et alo It is herehr certified 'that errorappears in Jche printed specification of 'the above numbered patentrequiring correction and thal Jthe said Letters .Patent should read ascorrected below.

Column l,n line 53g for adventagew reed .um disadvantages uw Signed thisllth of August l959 (SEAL) Attest:

KARL AXLINE ROBERT C. WATSON Commissioner of Patents Attesting OicerUNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent NQ, esoy/83April '7, 195e Charles En Hanks im inthe printed specification It ishereby certified that error appears ion and that the said Letters oftlfeA above numbered patent requiring correct .Patent should read ascorrected below.

Column l, line 53, for adventagess'f read 4diaetdvantage.e

Signed and this lltld day of August 19591,-

(SEAL) Attest:

KARL H, M1-LNE Attesting Ocer ROBERT C. WATSON Commissioner of Patents

